In order to manufacture a semiconductor integrated circuit, a wafer is generally subjected to various treatments such as film-forming, etching, oxidation, and diffusion treatments. In connection with a finer, higher integration of a semiconductor integrated circuit, a so-called clustered treatment system has been introduced to improve a throughput and a yield. Such a clustered system includes a plurality of treatment apparatuses for carrying out the same or different treatments, the apparatuses being connected to each other through a common conveying vessel. Thus, wafers can be continuously subjected to various treatments, without being exposed to an atmosphere.
In this type of treatment system, by using a conveying mechanism disposed in a conveying vessel, semiconductor wafers are taken out from a cassette vessel disposed on an inlet port, and are taken into the conveying vessel. The semiconductor wafers taken into the conveying vessel are aligned with each other by an orienter, and then loaded in a load lock vessel, which is capable of forming a vacuum, by means of the conveying mechanism. The wafers loaded in the load lock vessel are loaded in a common conveying vessel of vacuum atmosphere, by means of another conveying mechanism disposed in the common conveying vessel. A plurality of treatment vessels of vacuum atmosphere are connected to each other to surround the common conveying vessel. Wafers are sequentially sent into the respective treatment vessels via the common conveying vessel, so that the wafers are continuously subject to various treatments. The treated wafers are reversely sent along the original course, and are contained in the original cassette vessel.
As described above, the treatment system of this type generally includes a plurality of conveying mechanisms. Wafers are automatically delivered, received, and conveyed by means of the conveying mechanisms.
For example, the conveying mechanism is formed of an articulated arm capable of moving both horizontally and vertically, bending, stretching, and turning. A wafer as an object to be conveyed, which is directly held by a pick disposed on a distal end of the arm, is conveyed to a predetermined position. Japanese Patent Laid-Open Publication No. 338554/1994 shows an example of such a conveying mechanism including two conveying arms capable of bending and stretching, disposed on a rotary table. The conveying arms are spaced apart from each other, and have perpendicular rotational axes, respectively. Another example of such a conveying mechanism is shown in Japanese Patent Laid-Open Publication No. 284049/1999, which includes a first arm which can turn on a table, and two second arms connected to a distal end of the first arm which can rotate in a horizontal plane independently from each other.
In these conveying mechanisms, during an operation thereof, an arm, a pick, and a wafer should be prevented from colliding or interfering with other members. Further, a wafer disposed on a certain position should be properly held, and the wafer should be conveyed to a desired position and positioned thereon with high positional accuracy of within, e.g., ±0.20 mm.
Thus, when a system including a conveying mechanism is fabricated, or drastically remodeled, a so-called teaching operation is carried out for inputting positional information represented by position coordinates to a controller, e.g. a computer. As the positional information, important positions such as a position where a wafer is delivered and received in a channel where a pick is moved in the conveying mechanism, are input to the controller.
When the system is fabricated or drastically remodeled, errors such as a process error and an assembly error of members, and an error caused by a resolution of an encoder inevitably occur. Thus, prior to the teaching operation, a reference position should be corrected (calibrated) by calculating a deviation amount which indicates how an operational coordinate showing an actual position of the conveying mechanism in operation deviates from a theoretical coordinate determined by designing dimensions of a treatment system. The correction is executed relative to all the directions where the conveying mechanism moves. That is, when the conveying mechanism moves horizontally, a reference position is corrected in the horizontal moving direction (also referred to as “X direction”). When the conveying mechanism moves vertically, a reference position is corrected in the vertical direction (Z direction). At the same time, reference positions are corrected in a horizontal turning direction where an arm turns (θ direction), and in a horizontal operating direction where the arm bends and stretches (R direction).
An example of a method of correcting a reference position is disclosed in the specification of U.S. Pat. No. 5,535,306, in which coordinates of a conveying arm are automatically corrected in the R direction and the θ direction. According to this method, a wafer holding part of a conveying arm disposed in a common conveying vessel moves therein. A position of the wafer holding part is detected by a light sensor having a vertical optical axis. In this case, the reference positions can be corrected only in the R direction and the θ direction with one sensing light beam of the sensor. That is, reference positions cannot be corrected in other directions, i.e., the X direction and the Z direction.
Another example of a method of correcting a reference position is disclosed in Japanese Patent Laid-Open Publication No. 254359/1999, in which the respective coordinates of a conveying arm in the R, θ, and Z directions are automatically detected, and reference positions are corrected based on these coordinates. In this method, a wafer holding part of a conveying arm disposed in a common conveying vessel moves in a treatment vessel or the like connected to the common conveying vessel. The respective coordinates of the wafer holding part in the R, θ, and Z directions are automatically detected by a first light sensor having a vertical optical axis and a second light sensor having a horizontal optical axis. In this case, the reference position can be corrected only in the R, θ, and Z directions, by means of the two sensing light beams of the light sensors, and a correction of a reference position in the X direction cannot be carried out.